1. Field of the Invention
The present invention is directed to a self-supporting cantilevered arm attached to a vertical mast to provide a cantilever structure especially adapted to support lighted signals along a railroad, such as traffic signals at a grade crossing.
2. Description of Related Art
Some type of warning system is usually installed at grade crossings where railroad tracks cross a road or highway. Increasingly, governments require overhead warning lights and signals. State Departments of Transportation now typically require a set of signal lights in each lane of the roadway.
In addition, each light in such a warning display must be equipped with a restrictive filter that concentrates the light to be seen in the lane directly in front of the light. To meet this requirement, lens filters for overhead lights must direct the light into a cone having a total horizontal field of twenty degrees (20.degree.), i.e. ten degrees (10.degree.) to the left of the light's center line and ten degrees (10.degree.) to the right of the light's center line, and a total vertical field of thirty-two degrees (32.degree.) down, i.e. zero degrees (0.degree.) up from the light's center line and thirty-two degrees (32.degree.) down from the light's center line. To ensure that the resulting cone of light will be seen by a car in the lane in front of the light, the structure upon which the light is mounted must be fixed and not subject to excessive deflection.
The American Association of State and Highway Transportation Officials (AASHTO) also sets deflection and other performance criteria for such cantilevers. In a wind of 130 miles per hour (209 kph), that is, a Beaufort number greater than 17, a cantilever boom of any length is allowed to have a maximum vertically downward deflection of six inches (6") (15.24 cm) from the unstressed equilibrium position, and a maximum horizontal angular deflection of three degrees fifteen minutes (3.degree. 15') in front of or behind the equilibrium position. These are the maximum deflections that will allow the cone of light from each warning lamp to be seen by a driver in the lane beneath it. Such structures also should withstand snow and ice loading of three pounds per square foot and a live load of 500 pounds (186.5 kg) at the end of the cantilevered boom without exceeding these same deflection standards.
One approach to meeting such requirements is to build an elevated truss spanning the entire roadway and supported at both ends. This solution has generally not been commercially undertaken because it is too costly, since the truss need not cross the entire roadway but only the lanes of traffic that travel in one direction.
One prior art approach to the problem of providing overhead warning signals at grade crossings is a cantilever having a vertically disposed mast fixed to a supporting pad, such as a poured concrete pad embedded in the ground, and having a cantilever arm attached to the mast. The cantilever arm includes three main members, two of which form a triangular base disposed in a horizontal plane, and the third member is spaced above these two bottom members. Reinforcing members maintain the orientation of the three main members. Such structures, however, are not self-supporting. Instead, much of the support for the cantilevered arm comes from a pair of tensioned cables strung between the top of the mast and cable-retaining fixtures near the convergent end of the bottom members of the arm.
Such structures typically will not hold a 500 pound (186.5 kg) live load and are only good for a two-lane road at best. They will not support a walkway or a worker. Instead, whenever the warning lamps or signals require any maintenance or inspection, a hand crank is turned, swinging the cantilevered arm into a position parallel with the road and a supplied ladder is placed against the cantilevered arm at the spot where work is needed. Naturally, a worker must climb down the ladder and move it in order to work on more than one lamp or signal fixture.
Furthermore, the cables that hold the cantilevered arm must be tightened annually because they stretch. Eventually, the cables will stretch beyond their limit and must be replaced. If this maintenance is neglected, the result could be the collapse of the cantilevered arm.
Another proposed solution to the problem of providing overhead warning systems at grade crossings is a cantilevered arm consisting of a rectangular frame with some reinforcing members between the two long sides of the rectangle. The frame is oriented in a vertical plane and cantilevered from a mast. Such a cantilevered arm may include a walkway, allowing the maintenance worker to walk along inside the length of the arm to work on the lighting and warning fixtures. The space for walking inside the arm is small and restrictive. Moreover, the cost of this type of structure is high.
Accordingly, there is a significant need for an overhead cantilevered boom warning signal carrier for railroad grade crossings that can be extended to a length of forty feet (40') (12.19 km) while maintaining the necessary strength; that can carry a 500 pound (186.5 kg) live load at the end of the arm without deflecting more than six inches (6") (15.24 cm) downward; that will withstand a 130 miles per hour (209 kph) wind with a maximum vertical deflection of six inches (6") (15.24 cm) downward and a maximum horizontal angular deflection of plus or minus three degrees fifteen minutes (3.degree. 15') from equilibrium at the end of a forty foot (40') (12.19 m) long cantilever boom; that includes a catwalk for allowing easy access to the lighting fixtures; that allows the worker ready access to the lighting and warning fixtures without any superstructure members to crawl around or under; that remains in place over the roadway during maintenance on the warning fixtures, obviating the need to rotate the cantilevered boom; that weighs less than cantilevered booms of the prior art; and that is less expensive to manufacture, transport and erect than cantilevered booms in present use.